import java.util.Arrays;

public class MyStack2<E> {
    public Object[] elem;
    public int usedSize;

    public MyStack2(){
        this.elem=new Object[10];//初始设定为10个空间
    }
    public void push(E val){
        //判断是否满了
        if(isFull()){
            //扩容
            elem= Arrays.copyOf(elem,2*elem.length);
        }
        elem[usedSize]=val;
        usedSize++;
    }
    public boolean isFull(){
        return usedSize==elem.length;
    }
    public E pop(){
        if(empty()){
            return null;
        }
        E oldVal=(E)elem[usedSize-1];
//        elem[usedSize-1]=null;//如果是引用类型要置空
        usedSize--;
        return oldVal;
    }
    public E peek(){
        if (empty()) {
            return null;
        }
        return (E)elem[usedSize-1];
    }
    public boolean empty(){
        return usedSize==0;
    }
}
